The mu rhythm is a spontaneous electroencephalographic pattern which can be recorded over human somatosensory cortex in the absence of movement. Power spectral analysis across 50 sessions of EEG feedback training showed that mu activity could be systematically enhanced, whereas the 12--14 c/sec component of low voltage fast activity could not be modified. Results indicate that long-term modification of at least one normal cortical rhythm, initially present in the human EEG, is possible with feedback training but that training cannot produce such a rhythm.